Socket Weld vs Threaded Flange
The two small-bore options in ASME B16.5: same dimensions, and a joint that either resists vibration or slowly gives in to it.
- Both published
- NPS 1/2 – NPS 3 (small bore)
- Joint
- Fillet weld vs NPT thread
- Expansion gap
- 1/16 in, socket weld only
- Dimensions
- Identical — shared B16.5 table
Short answer. Both are small-bore flanges sharing one set of B16.5 dimensions. A socket weld flange takes the pipe into a counterbored socket and is fillet-welded outside; a threaded flange screws on with no welding. Socket weld wins on strength, vibration resistance and leak-tightness; threaded wins where hot work is impossible. In new construction, socket weld is the default and threaded is the exception.
ASME B16.5 tabulates one set of outside diameters, thicknesses, bolt circles and bolt patterns per class per size, and every flange type in that class uses them. A socket weld and a threaded of the same class and size have identical bolting and are fully interchangeable at the joint — the difference is entirely in how each attaches to the pipe.
The 1/16 inch gap
A socket weld joint is assembled by inserting the pipe to the bottom of the socket and then backing it out about 1/16 in before welding. Without that gap, the pipe bottoms out and thermal expansion of the weld drives a crack into the fillet root. It is the single most common socket weld defect, and it is invisible once the joint is welded.
The gap is also a crevice — a dead volume where the fluid does not move. That rules socket welds out of sanitary service, and makes them a poor choice in chloride-bearing or otherwise crevice-corrosion-prone systems.
Dimensions are given in inches, with the millimetre equivalent beneath in grey.
| NPS | Cl 150 OD | Cl 150 hub | Cl 150 bolting | Cl 300 OD | Cl 300 hub | Cl 300 bolting |
|---|---|---|---|---|---|---|
| NPS 1/2 | 3.5088.90 mm | 0.6215.75 mm | 4 × 1/2" | 3.7595.25 mm | 0.8822.35 mm | 4 × 1/2" |
| NPS 3/4 | 3.8898.55 mm | 0.6215.75 mm | 4 × 1/2" | 4.62117.35 mm | 1.0025.40 mm | 4 × 5/8" |
| NPS 1 | 4.25107.95 mm | 0.6917.53 mm | 4 × 1/2" | 4.88123.95 mm | 1.0626.92 mm | 4 × 5/8" |
| NPS 1 1/4 | 4.62117.35 mm | 0.8120.57 mm | 4 × 1/2" | 5.25133.35 mm | 1.0626.92 mm | 4 × 5/8" |
| NPS 1 1/2 | 5.00127.00 mm | 0.8822.35 mm | 4 × 1/2" | 6.12155.45 mm | 1.1930.23 mm | 4 × 3/4" |
| NPS 2 | 6.00152.40 mm | 1.0025.40 mm | 4 × 5/8" | 6.50165.10 mm | 1.3133.27 mm | 8 × 5/8" |
| NPS 2 1/2 | 7.00177.80 mm | 1.1228.45 mm | 4 × 5/8" | 7.50190.50 mm | 1.5038.10 mm | 8 × 3/4" |
| NPS 3 | 7.50190.50 mm | 1.1930.23 mm | 4 × 5/8" | 8.25209.55 mm | 1.6942.93 mm | 8 × 3/4" |
Socket weld flanges are published in the small sizes shown; the same table serves threaded flanges.
Socket weld
- Good static strength in small bore, and easy to fit up
- No weld metal can protrude into the bore
- Needs a 1/16 in expansion gap at the socket bottom to avoid weld cracking
- The gap is a crevice — poor choice for corrosive or sanitary service
Threaded
- No welding, so usable in flammable or explosive areas
- Fast field installation with hand tools
- Not for cyclic, high-temperature or vibrating service — the thread loosens
- Thread root reduces the effective wall; needs thick-wall pipe
Vibration decides most of these
Small-bore connections off pumps, compressors and steam headers fail by fatigue at the joint more often than by anything else. A welded socket has no mechanism to loosen; an NPT thread does. Where a small-bore line is cantilevered off vibrating equipment, socket weld — properly gapped and, ideally, braced — is the answer, and a threaded connection is a future leak.
Dimensions: socket weld flanges · threaded flanges · slip-on vs threaded · end connections.
Common questions
Why do socket weld joints need a gap?
To let the weld and the pipe expand without loading the fillet root. Pipe bottomed out in the socket puts the shrinkage strain straight into the weld and cracks it. About 1/16 in of clearance is specified.
Are socket weld and threaded flanges the same dimensions?
Yes. B16.5 publishes one set of dimensions for slip-on, threaded and socket weld flanges, including the same length through hub.
What sizes are socket weld flanges available in?
Small bore — commonly NPS 1/2 through NPS 3. Above that the socket becomes impractical and slip-on or weld neck is used.
Which is better for steam tracing and instrument lines?
Socket weld, in almost every case. Those lines are small, hot and often vibrating, which is exactly where threads loosen.